Design, Implementation, and Experimental Characterization of a non-LTI Electrically Small HF Antenna with Improved Bandwidth-Efficiency Product

We present the design and experimental characterization of a non-linear-time-invariant (i.e., non-LTI) electrically small antenna (ESA) operating at the high-frequency (HF) band. This non-LTI ESA is designed to overcome the problems associated with the fundamental bandwidth-efficiency product limitations that govern the performances of passive, linear-time-invariant (LTI) ESAs. The antenna comprises an electrically small radiator and a class-E switch-mode inverter and can transmit multiple types of digitally modulated waveforms in the HF band. Due to the large wavelength of electromagnetic waves in the HF band, practical HF antennas are usually electrically small. Because of their limited dimensions, passive electrically small antennas suffer from large quality factors and are hard to impedance match over wide bandwidths. The physical limitations that govern the bandwidth-efficiency products of ESAs are well known (e.g., the Chu, Wheeler, and Gustafsson limits) and have been thoroughly investigated and confirmed by many ESA designs. Different techniques, such as non-foster impedance matching, active matching, and direct-antenna modulation among others, have been investigated to overcome these limitations. These techniques do not violate the laws of physics. Rather, they violate the linear-time-invariance or passive operation assumptions used in the derivation of the fundamental physical limits. In a recent work (N. Behdad, D. Ludois, M. Mirmozafari, and M. Liben, “A class-E, switched-mode, non-LTI electrically-small transmit antenna design for overcoming the fundamental bandwidth-efficiency product limits,” 18th European Conf. Antennas Propag., Glasgow, UK, 2024), our team presented the design of a non-LTI ESA that integrates a switched-mode amplifier with an electrically small radiator. Through simulations, we demonstrated that this ESA is expected to be capable of radiating wideband, digitally modulated waveforms with bandwidth-efficiency products that exceed the fundamental limits. In this work, we present experimental results of a fabricated prototype of this non-LTI ESA. The prototype consists of a 1-m long monopole as the radiating element integrated with a class-E switch-mode circuit employing a GaN transistor switch. Digitally modulated waveforms are directly generated at the antenna stage by controlling the duty cycle of the gate driver signals applied to the switch-mode circuit. Amplitude, phase, and frequency control can be achieved by controlling the duty cycle, timing, and frequency of the gate driver signals. In this non-LTI ESA, the tasks of RF carrier generation, modulation, amplification, and radiation are all performed in a single device and only DC bias and gate driver control signals are supplied to the system. We have conducted system-level over-the-air (OTA) experiments using the non-LTI ESA as the transmitter and a conventional receiver system. The bit-error-rate (BER) and error vector magnitude (EVM) of the communication system are measured for a link operating at 10 MHz. We have also conducted similar OTA experiments using a conventional LTI ESA transmitter that uses the same radiating element in conjunction with an impedance matching network and a commercially available power amplifier. We will present the results of these two experiments and compare the bandwidth-efficiency product of the non-LTI ESA with that of its LTI counterpart. Finally, we will also present the initial designs of the next generation of this non-LTI ESA employing a closed-loop control system allowing for more efficient pulse shaping of the radiated siganls.